One obvious disadvantage of this approach, is that I need to connect and disconnect in every function. A possible solution to this, would be having a function f.ex dblink_exists('connection_name') that returns true/false depending on whether the connection already exists.
Can't you do this already?
SELECT 'myconn' = ANY (dblink_get_connections());
A dedicated function might be a tad faster, but it probably isn't going to matter compared to the overhead of sending a remote query.
I agree. The above is about as simple as SELECT dblink_exists('dtest1'); and probably not measurably slower. If you still think a dedicated function is needed, please send the output of some performance testing to justify it.
If you really want the notational simplicity, you could use an SQL function to wrap it:
CREATE OR REPLACE FUNCTION dblink_exists(text) RETURNS bool AS $$ SELECT $1 = ANY (dblink_get_connections()) $$ LANGUAGE sql;
contrib_regression=# SELECT dblink_exists('dtest1'); dblink_exists --------------- f (1 row)
I guess it might be worthwhile adding the SQL function definition to dblink.sql.in as an enhancement in 8.4.